Download Chapter 5

5
Life’s Border:
The Plasma Membrane
I. Introduction (Sections 5.1 and 5.2)
A. Importance of the plasma membrane (seen in
malfunctioning transport in cystic fibrosis)
B. Overview of major functions:
1. Regulates what goes into and out of a cell.
2. Communication with other cells
II. Four Components of the Plasma Membrane
(Section 5.3)
A. Phospholipid Bilayer
1. Phospholipids—two fatty-acid chains and a polar
phosphate group attached to glycerol: Figure 5.1a
2.Arrangement of phospholipids in water (two layers,
heads pointed out, tails pointed in): Figure 5.1b
3.Permeability of bilayer—lipid center is a barrier to
passage of large hydrophilic molecules, but it allows
nonpolar, hydrophobic molecules to pass. (Interactive
Activity 1)
B. Cholesterol (prevents passage of some small molecules
and adds fluidity): Figure 5.2
C. Proteins—integral (span entire membrane) and
peripheral (lie on either side) with diverse range of
functions: Figure 5.2
1. Structural support—attach to cytoskeleton
2. Recognition—helps immune system determine
self from foreign (that’s why we reject transplants
3. Communication—receptors and binding sites
(Interactive Activity 2)
4. Transport—allow molecules to pass
D. Glycocalyx Figure 5.2—sugar components protruding from lipids and
proteins, functions:
1. Binding sites for proteins in communication, and recognition
2. Lubricate cells
3. Stick cells down
III. Moving Materials In and Out: Diffusion
and Gradients (Section 5.4)
A. Random Movement and Diffusion: Figures 5.3 and 5.4
1. Diffusion = movement of molecules from region
of higher to lower concentration
2. Concentration gradient = difference between the
highest and lowest concentration of a solute, like
bike coasting downhill, the tendency is for
molecules to travel from high to low concentration.
Diffusion
Random
Movement
B. Diffusion through Membranes:
C. Permeability verses semi-permeability
D. Osmosis = net movement of water across a semipermeable membrane from an area of lower solute
concentration to higher solute concentration: Figure
5.5 animation
E. Importance of osmosis to membrane function in
animals (drinking sea water) and plants (turgid
pressure): Figure 5.6
OSMOSIS
Hypertonic Solution – More
water inside cell than outside
Plasmolysis
Plasmolysis
Cells shrink
and die
Iso-osmotic Solution – same concentration of
water inside and outside cell
Animal cell
Plant cell
No net gain or
loss of water
Hypotonic Solution – more water
outside cell than inside
Cytolysis –
cell swells and
bursts
Builds up turgor
pressure – cell
becomes stiff,
keeping plant
upright
IV. Two Main Types of Transport across Cell
Membranes (Section 5.5)
A. Passive Transport
1. Simple diffusion (water, gases, fat-soluble) membrane is
permeable, so they travel down concentration gradient and
enter without energy output by cell: Figure 5.7a
2. Facilitated diffusion: (larger polar molecules) membrane is
impermeable, so even if they want to travel down the
concentration gradient they can’t without help from a
membrane channel (transport protein): Figure 5.7b
B. Active Transport—If molecules have to pass across the membrane
up their concentration gradient, they cannot use the energy of
diffusion, but must expend energy (ATP): Figure 5.7c
Na-K pump Figure 5.8
V.
Getting the Big Stuff In and Out (Section 5.6)
A. Exocytosis: Figure 5.9—movement of materials out of the cell by
fusion of vesicles with the plasma membrane (export or removal of
wastes in single-celled organisms)
B. Endocytosis—Infolding of the plasma membrane to bring
large materials into the cell
Pinocytotic vessicle
1. Pinocytosis, “cell drinking”—water and solvents are
enclosed in invaginating vesicle, used in digestive
tract: Figure 5.10a
D. Receptor-mediated endocytosis—more specific with
receptor capturing ligand and concentrating into an
invaginating pit: Figure 5.10b
E. Phagocytosis, “cell eating”—How the human immune system
ingests whole bacteria or one-celled creatures eat: pseudopodia,
Figure 5.10c (Interactive Activity 4 & 5)